A plumb-bob or a plummet is a weight with a pointed tip on the bottom that is suspended from a string and used as a vertical reference line.This
instrument has been used since the time of the ancient
Egyptians by bricklayers,
masons, and carpenters to ensure that their constructions are "plumb", or perfectly upright. It may also be used in surveying to sight a point on the ground that is not readily visible. Small plumb bobs are included in the kits of various instruments such as levels and
theodolites. They are used to set the instrument exactly over a fixed
datum marker, prior to taking fresh readings.
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